Used in describing Daguerreotypes, Ambrotypes, Tintypes, and other Formats
The "plate sizes" used in referring to 19th-century daguerreotypes, ambrotypes, tintypes, and other photographs stem originally from the daguerreotype, for which the first plates manufactured were approximately 6.5 x 8.5 inches (16.5 x 21.5 cm) in size. Smaller size daguerreotypes were then defined in terms of how many plates of the smaller size could be cut from such a "full" or "whole" plate. These plate sizes became standardized and were subsequently used for the photographic formats which followed on the daguerreotype. Plate sizes are still the standard method for referring to the dimensions of these 19th century images.
An image's plate size refers to the size of the complete image plate, including portions which may be covered by its mount, and not just the visible portion. In general, for an image housed in an American-style case, the dimensions of the brass mat and "protector" will be approximately equal to those of the plate and can be used to determine plate size (see illustration below).
Note that plate sizes are approximate. Besides variation in the sizes of plates as they came from the manufacturer, finished plates were often trimmed on one or more edges by the photographer to allow for positioning under the mat. Any shifting of the plate under mat from exact center - to better center or otherwise improve the composition seen through the mat opening - would normally cause one or more plate edges stick out beyond the mat, and the photographer would have to trim off these projecting portions to allow the image package to fit in a standard-sized housing.
Plates were also trimmed down to oval or other shapes in order to fit them into odd-sized cases, often considerable reducing their size. In these cases the plate size is normally given as the size of the plate before trimming, assumed to be the smallest standard plate size from which the image could have been cut. To the opposite effect, small size plates were sometimes overmatted (placed under a larger-size mat with small opening or "window") in order to enhance the appearance of the finished package.
Other Plate Sizes:
Images in European-style Mounts:
The plate size of a daguerreotype, ambrotype, or similar image in a European-style housing is not equal to the size of its mount, and cannot be determined by examining the back of the mount or any openings or ridges found there. The most accurate way to determine the size of an image in a European mount, without opening it, is to measure the visible area of the plate. The plate's actual size will normally be the smallest standard size that would allow for such a visible area. European plates were often framed to show more of the image, to the point that the edges of the plate and plate hallmarks are sometimes visible even when housed in the mount.
Copyright 2007-2008 by Christopher Wahren